Spoil Me Rotten for the $2000 Challenge

I'm feeling inspired by this month's issue of GRM with the Pink Panther. Where would you put a ducktail spoiler on this silly Challenge station wagon? What angle of attack would be a good place to start?

• Optimized for auto-cross - low speed and max downforce. Hence ducktail. I'm open to other options though.
• Challenge car (no rules), but would like to fit SCCA E-Mod class. Rules for that:
  • If mounted at base of rear window ("trunk spoiler") it can extend 10" from base
  • If mounted at back of the roof ("roof spoiler") it can extend 7.5" from base
  • Angle of attack is free
• Must be easily removable for drag racing
• Built from free materials, such as scrap aluminum rain gutters, coroplast yard signs, dumpster plywood, etc.

I feel like with the "trunk spoiler" I could go all the way down to the rear fender flares to scoop up some extra side air and divert it with end plates. And yes I'm planning a splitter build too.

Put the spoiler as far rearward as you dare. Make it bigger than 10" and then cut it down for SCCA legality later (or build two, one SCCA legal and one HUGE). Angle ----> about 20 degrees from vertical.

In reply to stafford1500 :

How HUGE are we talkin? All the way up to roof level like in the picture?

Higher than roof level. Make it big.

Also, I should have some testing on my plywood wing in the next few weeks so watch for that.

World Of Outlaws!

maschinenbau said:

In reply to stafford1500 :

How HUGE are we talkin? All the way up to roof level like in the picture?

A sheet of plywood is ONLY 4ft*8ft. You may need some extra bracing and higher rear spring rates. Power may also come into play.

I don't need to see what's behind me anyway because I'm a forward-thinking individual with my eyes on the prize.

stafford1500 said:

maschinenbau said:

In reply to stafford1500 :

How HUGE are we talkin? All the way up to roof level like in the picture?

A sheet of plywood is ONLY 4ft*8ft. You may need some extra bracing and higher rear spring rates. Power may also come into play.

I’ll suggest that ‘wires in tension’ is probably the cheapest-lightest way to brace the top from changing angle.  But you’ll probably want to reinforce the bottom some too, where the load/moment is transferred to the chassis.

Only problem I found last night is the rear hatch window goes pretty far down, so the base of the spoiler would be mounted on glass. Maybe I can buy a bunch of cheap phone mount suction cups to distribute the load? It would be easily removable too.

maschinenbau said:

Only problem I found last night is the rear hatch window goes pretty far down, so the base of the spoiler would be mounted on glass. Maybe I can buy a bunch of cheap phone mount suction cups to distribute the load? It would be easily removable too.

do you feel comfortable standing on your window?  or you and a buddy standing on your window?  How hard is it to track down another hatch glass?

I'd guess the body outside the rear hatch is particularly strong... you probably just need some kind of triangle or maybe "L" bracket mounted to that which the spoiler bolts into... this needs to be removable for drags, so it's not going to affect the hatch.  Although, making it "part of the car looks" would take some thinking. 

Car is 6ft wide, spoiler will probably be like 10-12" tall, assume .12 to .20 psi at autocross speeds produces 100 to 175 lbs? Is that realistc at all? I spread that out over 4 or 5 suction cups and that's only 30 lbs per mount. Plus a lot of the load will go into tension cables that can hook over the leading edge of the hatch, like a bike rack. Just kinda spitballing here. 

I have a spreadsheet that I made. A 6 sqft wing at 200 ish lbs of downforce at 60 mph needs a coef of lift greater than 3 (I have read that fsae rear wings average around cL of 3).

So it's safe to say you will make less downforce than that. Or that you need a much bigger wing.

Mine's not a wing though, it's a ducktail spoiler. Not sure how to compute lift coef for that.

In reply to maschinenbau :

I don't think you have to compute the cL for it. Just to know that it is "less".

the challenge is that the book that I have (Competition Aerodynamics)... a) isn't accessible to me right now; b) provides coefficients based on 'vehicle cross-sectional area' and a set height and variable angle.  Which, makes a number of assumptions.  Hucho/SAE's 'Aerodynamics of Road Vehicles' is the reference for that, and supposedly has more data... but I haven't picked it up yet.

and, I'd probably err on the side of having wires bracing the spoiler both "fore" and "aft"... so that way braking and or "surface imperfections" don't cause the spoiler to swing forward

Just a little information that might help. The general ratio of downforce to drag for a spoiler is 2:1 for the sort of configuration you are looking at.

Bracing the top edge with wires is cheap and easy and lighter than tubing. If you can make a small flange reciever at the bottom of the hatch, then you can attach as big a spoiler as you want, depending on the rules or lack of them.

I really don’t like mounting it on glass, regardless of the actual downforce. 

Also disagree with your math a little. 150- 175 lbs distributed over 5 suction cups is still 150- 175 lbs on a single piece of glass. 

You’ve still got that much force pushing at the bottom edge of the glass. 

My shoes are 4.5 x 12.   2 of them total 0.75 SF. 

(5) 3” diameter suction cups = 0.245 SF surface area. 

I wouldn’t stand on your glass. 

In reply to SVreX :

Good point. I'll need to think more about the structure first.

What if the spoiler just went across the glass but was not mounted to it? Could you make some spoiler end plates to fasten the wing to the frame of the hatch instead? in a side profile maybe it would look something like a WRC focus wing, but with a sheet of material inside instead of an airfoil?

I assume you'd need it to still seal to the glass to make downforce, but you wouldn't need to worry so much about cracking the glass then maybe?

klodkrawler05 said:

What if the spoiler just went across the glass but was not mounted to it? Could you make some spoiler end plates to fasten the wing to the frame of the hatch instead? in a side profile maybe it would look something like a WRC focus wing, but with a sheet of material inside instead of an airfoil?

I assume you'd need it to still seal to the glass to make downforce, but you wouldn't need to worry so much about cracking the glass then maybe?

You'll still make downforce without sealing the spoiler to the glass.  It probably just won't be as much.  How much less, though, is probably hard to guess.  You could probably make up some of the difference by adding a second layer from the gap back 20% of the "height" of the spoiler and shape it some on the noise to try and generate some suction on the bottom side to make up for it.